1. Field of the Invention
The present invention relates to a phase modulator and an optical modulation device for modulating light.
2. Description of the Related Art
Mach-Zehnder interferometers (MZI) have been conventionally used, for example, to decode a phase-modulated optical signal. In optical interferometers such as MZI, dither signals have been used to control the phase modulation amount and to stabilize the operation point (see, for example, Patent Document 1 and Patent Document 2 cited below).
As a small-phase-modulation element that uses dither signal, a phase modulator of which phase modulation amount can be adjusted by applying heat by a heater, and a phase modulator that uses the electro-optical effect of a non-linear optical crystal such as LiNbO3 have been used. Furthermore, a technology has been disclosed in which a liquid-crystal optical modulation element is inserted as a phase modulator in either one of the arms of the MZI (see, for example, Patent Document 3 cited below).
[Patent Document 1] Japanese Patent Application Laid-Open Publication No. 2008-271028
[Patent Document 2] Japanese Patent No. 3210061
[Patent Document 3] Japanese Patent Application Laid-Open Publication No. 2008-122856
However, the phase modulator that uses the heating by the heater described above requires current drive, and thus has a problem of large power consumption (P=I2R, where I is current and R is resistance) during normal operation. On the other hand, the phase modulator that uses the non-linear optical crystal requires a high drive voltage of 10 volts or more, and thus requires a high-voltage-resistant drive circuit and has a problem of large power consumption (P∝fV2, where f is frequency and V is voltage).
Regarding the phase modulator that uses the liquid-crystal optical modulation element described above, the liquid-crystal cell needs to be thick to control the phase modulation amount for one wavelength (2π) or more. On the other hand, the thicker the liquid-crystal cell is, the slower the response speed thereof becomes, and thus the liquid-crystal cell becomes incapable of responding to the small-phase-modulation signal of several kHz for small modulation signal (dither signal). As a result, such a phase modulator cannot stabilize the operation point, and thus cannot perform stable phase modulation.
To solve the above problems, the present invention aims to provide a phase modulator and an optical modulation device with reduced power consumption. The present invention also aims to provide a phase modulator and an optical modulation device capable of stabilized phase modulation.